Actually, Darin's behaviour in the minutes after the murders proves to me he cared a lot more for his precious Darlie than those boys.
why were neither parent at either boy kissing, hugging, telling them to "hang on love, stay with me, i love you..."
Disturbing behavior from both of them! I'm not sure Darin's behavior was about actually "caring" for Darlie as much as covering his own butt. Both showed great emotional detachment from their children. I see them both as extremely immature and criminally self-involved. I believe whatever went on between them that night, neither could refocus their attention or concern on their children, even as they lay dying in pain.
It seems to me the kids fit in their lives only when they served a selfish purpose for the immature and self-obsessed parents (glue for the rocky marriage and perfect props for the appearance of the perfect "successful" family).
The perfect storm happened in the convergence of three factors:
1). Darlie's perceived rejection/abandonment by Darin that evening
2). A raging Borderline Personality Disordered temper tantrum caused by above
3). Fueled to the max by rage-invoking diet pills
Those factors, plus Darin probably having seen enough of her bull chit temper tantrums to be somewhat immune to them and maybe staying upstairs with door closed rolling his eyes while she raged until it was too late are what I think brought about this disaster.
That's my best guess anyway.